There is little doubt in the engineering and science communities that electric cars will play a significant role in the future of the automotive industry.As a result,resources are being devoted to creating better energy storage systems for such cars.While there are nearly endless chemical techniques for battery technology,researchers at General Motors and France’s Laboratoire de Reactivite et de Chimie des Solides see great potential in lithium-ion batteries.The lithium battery is already commonly used for portable electronic devices but,to make the jump to the electric vehicle market,quite a few improvements will be necessary.
One way to improve lithium-battery technology is to replace traditional carbon anodes with negative electrodes made of materials that are reduced by lithium to form a metal and a corresponding lithium compound.Efficient conversion reaction electrodes have dramatically increased the capacity of batteries,but they suffer from poor energy efficiency,mostly due to a reversibility loss in the redox reactions.The reversibility is diminished when the battery undergoes structural changes during the process of charging and discharging.
French scientist Luc Aymard led a research team that developed new conversion reaction electrodes that used metal hydrides—compounds with a metal covalently bonded to hydrogen atoms—to overcome such problems.Incidentally,metal hydrides may also be useful for fuel-cell technology,which is another candidate for powering electric vehicles.Although many metal hydrides are available,Aymard and colleagues chose the inexpensive magnesium hydride due to its advantageous electrochemical properties.The magnesium hydride electrodes mitigated battery cell polarization,meaning that the electrochemical process is highly reversible.
The reversible capacity is nearly the same as the discharge capacity: about three times better than current lithium-ion primary battery.The capacities are sustainable for at least 50 cycles,and the design can be tweaked to go much longer.Besides enhancing battery performance,a magnesium composite containing magnesium hydride,magnesium metal,and lithium hydride showed excellent hydrogen absorption and desorption properties.After only one hour at 100°C and 10 bars of hydrogen,95 percent of the reaction went to completion and absorbed hydrogen.As a comparison,bulk magnesium requires 30-40 bars of hydrogen,over 340°C and more than 6 hours to reach the same level of absorption.
The desorption process occurred in five hours at 200°C under a vacuum strength of 10 mbar,which is 140°C lower than conditions reported in previous research.Overall,magnesium hydride dramatically lowered the polarization of lithium-ion batteries without losing the advantage of having a high capacity.In addition,the relatively mild conditions required for hydrogen absorption and desorption point to potential benefits of using magnesium hydride in fuel cells.Aymard and colleagues propose that other metal hydrides may upgrade current battery and fuel cell technologies,as well.